Assignment 8_meta mafic rocks

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University of Cincinnati, Main Campus *

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3003C

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Geology

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Oct 30, 2023

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Petrology 3003C, 23SS Assignment 8 – Meta-basalts (AKA meta-mafic rocks) Your name(s) here: ___________________________________________________________ The goal of this assignment is to (1) have everyone look at a variety of metamorphosed mafic rocks in hand sample and thin-section, including greenstone, greenschist, amphibolite, and eclogite; (2) have everyone look at blueschist in hand sample and thin-section, (3) learn about what minerals are diagnostic of the greenschist, amphibolite, blueschist, and eclogite faces as expressed in metamorphosed mafic rocks; and (4) understand the connection between how the mineralogy of mafic rocks undergoing subduction zone metamorphism influences subduction zone magmatism. You can work in pairs or trios, or independently, of course. Part 1. Hand samples and mineralogy . In the classroom, there are trays of rocks that typify the various metamorphic facies we are working on…greenschist, amphibolite, blueschist, and eclogite. Have a look at these hand specimens. In these trays, you will find “information sheets” about each particular facies. Use these sheets to make a list of the minerals that characterize the different facies: [20 pts.] Greenschist facies– Amphibolite facies – Blueschist facies – Eclogite facies – Part 2. Petrography . Look at the following rocks and their thin sections: Greenschist – 88005 and 88007 . For these two rocks, write down two optical properties of chlorite, tremolite-actinolite, and calcite which allow you to positively identify these minerals. [20 pts] Greenstone – Look at two of the following rocks and their sections: MC8902, MC.94.L1, MC.95.1.3, MC.90.3.4, MC.95.1 . Answer the following questions for the rocks you selected: [2 pts each] 1. Is there relic pyroxene in the thin-section? 2. Has igneous plagioclase been replaced by epidote? 3. Is calcite present? 4. Is chlorite present?

5. Is a light to medium-green amphibole present? 6. Is there an opaque mineral? Amphibolite – Look at two of the following rocks and their sections: ARD.95.1, PS-1014, OC.95.1, MC.90.4.2, M-21, PS-1010 . Answer the following questions for the rocks you selected: [2 pts each] 1. Describe the amphibole which is present in each of your two sections (one description for each thin-section). 2. Is garnet present? 3. Is epidote (really, an epidote group mineral) present? 4. Is plagioclase present? 5. Is rutile or sphene present? 6. Is quartz or biotite present? Blueschist – Look at one of the following rocks and their thin-section: L9 (this is a very fine- grained rock), OBS-1, P-16, BSCCO-1 . [5 pts] 1. Does the rock you selected contain glaucophane? 2. What is the end-member formula of glaucophane? Look at OBS-2 . What are the major minerals in this rock? What metamorphic facies has this rock been in? [10 pts] Eclogite – Look at one of the following eclogites, F3-22, ECL-WIA-  , ECL-WIA-  , BAV-1 . Make of a list of all the minerals you can identify which are in your rock. Note: these particular eclogites have been overprinted and are composed of minerals which record a range of P and T (not just eclogite facies conditions). [10 pts] Extra credit : What is the mineral assemblage of sample PS-216? [5 pts] A rock with a fascinating texture! Sample MC.91.2.1B was collected from a mafic dike which cross-cuts Grenville (or Laurentian) basement gneiss located near Philadelphia; the gneiss has an age of about 1,000 Ma and records granulite metamorphism of Grenville age. This mafic dike is composed of garnet, green amphibole, brown biotite, large relic igneous plagioclase and small to large relic clinopyroxene, and an opaque. The plagioclase and pyroxene are relicts of the original igneous mafic dike. The other minerals record metamorphism in the amphibolite facies.

If you look carefully, you can find beautiful corona textures , where relic pyroxene is “ringed” by amphibole, and in some examples, “ringed” by amphibole + garnet, and amphibole forming an inner ring, surrounded by an outer ring of garnet. In other examples, pyroxene has been completed replaced by amphibole, and the amphibole has a corona of garnet.

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